1. Field of the Invention
The present invention relates to an optical disk drive, and more particularly, to an optical disk drive that writes data in an erasable optical disk.
2. Description of the Related Art
As a recordable optical disk, write once types such as CD-R and erasable types such as CD-RW are available. In order to overwrite data in an erasable type optical disk, a laser beam of which laser power changes in two steps, a write power Pw and an erase power Pe, is used. In this case, the recording layer of the optical disk is turned from a crystal state to an amorphous state by the laser beam of the write power Pw to form a recording mark, and the recording layer of the optical disk is turned from an amorphous state to a crystal state by the laser beam of the erase power Pe to erase the recording mark.
In the case of writing in an optical disk, the optimum write power Pwo and the optimum erase power Peo of the laser beam depend on the kind of the optical disk, the optical disk drive, and the recording speed. Accordingly, in order to set the optimum write power Pwo and the optimum erase power Peo that suit the combination of the kind of the optical disk, the optical disk drive, and the recording speed, a write power calibration operation called Optimum Power Control (OPC) is performed before the writing of data.
The conventional OPC operation of an erasable optical disk drive will be described below. On a recording face of the optical disk, as shown in FIG. 6, a data area to store various data therein and a power calibration area (PCA) that is a test writing region to set the optimum write power of the laser beam are provided. The PCA is provided at the most inner radius of the disk. The PCA consists of a test area and a count area. The test area is divided into 100 partitions, each partition consisting of 15 frames. One OPC operation is performed using one of the 100 partitions. A test signal of which laser power changes in 15 steps is recorded in 15 frames in a partition, each frame being recorded using one step of laser power. This test signal is an EFM modulated signal of a series of pulses having a time width of 3-11 times of a reference time width T (T=230 nsec, a cycle frequency of 4.32 MHz at a standard speed (single speed)). Recording marks having varying lengths of 9 kinds are written in the frame.
The test signal is reproduced by detecting the reflected light of a laser beam applied to these frames. A modulation factor m as an index of the amplitude of the reproduced signal is also measured.m=I11/Itop
As showed in FIG. 7, I11 is the amplitude of a detected signal generated by an 11T recording mark and a land (the portion between recording marks), and I3 is the amplitude of a detected signal generated by a 3T recording mark and the land. Itop is a reflectivity of the land portion. The modulation factor m depends on the write power Pw.
When the write power is low, the modulation factor m is small because the amplitude of the reproduced signal is small. The greater the write power Pw becomes, the greater the amplitude of the reproduced signal becomes, and consequently, the greater the modulation factor m becomes. When the write power becomes greater to a sufficient extent, the modulation factor m saturates. If an optical disk is recorded at the write power Pw around which the modulation factor m begins saturating, the reproduced signal contains the least jitter and/or error. This write power is determined to be the optimum write power Pwo.
This optimum write power Pwo is used as the write power for the writing of signals. The optimum erase power Peo is determined based on the optimum write power Pwo by multiplying a coefficient εo (erase power/write power ratio) recorded in the optical disk as ATIP information.Peo=εo×Pwo
There are two methods of overwriting: a method in which a written signal is erased and then, a signal is written and a method in which a signal is directly overwritten (direct overwrite). Most optical disk drives available in the market utilize the direct overwrite method and few optical disk drives utilize the erase and write method. The direct overwrite method inherits the following problems.
Ordinarily, the write power that best fits the optical disk drive and the optical disk (medium) is determined by the OPC. If an appropriate signal amplitude (modulation factor), an overwrite property, and an appropriate erasing durability are taken into account when the optimum write power is determined, no problem would occur as long as the optical disk is written and read by the same optical disk drive.
However, the erasable optical disk such as a CD-RW disk is a compatible medium. The optical disk that is recorded by an optical disk drive manufactured by a manufacturer may be overwritten by an optical disk drive manufactured by another manufacturer. In this case, if the write power by which the signal is written in the optical disk is too high (the modulation factor is rather too high), the signal will remain un-erased depending on the erase power to be used during the overwriting.
That is, on the overwriting, laser power of two kinds, the write power to write a recording mark and the erase power to form a portion (space) where no recording mark exists, is applied to an existing recording region to perform the overwriting. In the case that the existing signal written on a track is too great, the existing signal is not erased and remains in the erased region (space region). It may cause a too low C/N ratio of the reproduced signal after the overwriting and a too high error rate.
If the write power and the erase power are equally increased to avoid this problem, the erasing durability of the optical disk is lowered and the life of the optical disk is shortened. Additionally, since the overwrite properties becomes worse if an optical disk recorded by an optical disk drive is further overwritten by another optical disk drive, the error rate of the reading of a signal becomes worse.